The present invention relates to a method of applying a magnetic liquid to a long wide web, and more particularly relates to an improved method in starting the application of a magnetic liquid to a moving web by an extrusion application device or in resuming the application after the passing of the joint of two webs across an extruder head.
To conventionally manufacture a magnetic recording medium or the like, an agent which is to become a magnetic layer, a protective layer, a backing layer or the like is continuously and uniformly applied to a long wide web made of a polyethylene terephthalate or the like and having a thickness of several microns to several hundred microns, a width of several hundred millimeters to several thousand meters. The applied agent is dried, and the assembly of the dried agent and the web is wound around a reeling shaft. The assembly is cut into a plurality of slender pieces, each having a width equal to that of the magnetic recording medium or the like. Each slender piece is cut into segments, each having a length equal to that of the medium or the like, and each segment is wound around a spool or the like.
Various methods have been used in the past to apply agent to a web including roller application, gravure application, roller application-brass doctor, extrusion application, slide bead application and so forth. The method of extrusion application and the method of slide bead application have recently been used more widely than the other methods.
Methods of extrusion application have been disclosed in Japanese Patent Publication Gazettes Nos. 47-45523 and 54-7306, U.S. Pat. No. 3,227,136 and Japanese Laid-Open Gazette No. 58-189069. Japanese Patent Publication No. 58-39588 discloses extrusion coating and thereafter doctoring the coating to obtain predetermined coating thickness. A method of slide bead application type has been disclosed in Japanese Patent Laid-Open Gazette No. 48-98803. In these methods, a liquid in which a magnetic substance is dispersed is supplied to an extruder head by a constant quantity pump and then all of the liquid is applied to a web. The quantity V (cc./min.) of the supplied liquid is normally determined as follows: EQU V=a . s . w
In the above formula, a denotes a quantity (cc./m.sup.2) necessary to be applied to the web, s denotes the speed (m/min.) of the application, and w denotes the width (m) of the application. If the quantity V of the liquid supplied by the pump fluctuates, the quantity a applied to the web fluctuates too. In that case, the necessary quantity a is not maintained, so that a desired product is not made. When the applied liquid is scraped off by a doctor blade located downstream of the extruder head with respect to the direction of the movement of the web, the pressure on the doctor blade fluctuates because of the fluctuations in the quantity of the liquid applied by the extruder head, so that a steplike irregularity or the like is caused as to the thickness of the film of the applied liquid. For these reasons, the applied quantity of the liquid should be as constant as possible.
The fluctuation in the applied quantity of the liquid which produces an adverse effect as described above, is most likely to occur in the following cases:
(1) When the application of the liquid to the web is started: For instance, at the first time the application of liquid is stated, or at the time of resuming the application after its stoppage due to trouble or the like.
(2) When the application of the liquid is resumed after the joint of two webs has passed across the extruder head. (During passage of the joint across the extruder head, the extruder head is sufficiently separated from the webs so that the webs do not contact the extruder head.)
The method of passing the joint of two webs across the extruder head is now described. The length of each web is usually several hundred meters to several thousand meters. To continuously apply the liquid to plural webs, these webs are sequentially conjoined to each other as one web is sent after the other. To join the webs, the webs are overlaid on each other or butted to each other so as to be conjoined together by a two-sided-adhesive tape or the like. A method of butting webs to each other and conjoining them together has been disclosed in the Japanese Patent Laid-Open Gazette No. 55-142565. Since the joint of the webs has different levels due to their overlay, or to the presence of the tape or the like, the joint comes into contact with the extruder head. In order to avoid this contact, one of the procedures described below is usually taken.
(i) The gap between the web and the extruder head is made larger by several ten microns to several hundred microns at the time of passing the joint across the extruder head as compared to the gap which exists at the time of normal application.
(ii) The extruder head is thoroughly separated from the webs, and the joint of the webs is then passed across the extruder head. After that, the application is resumed.
When the application of liquid is started for the first time or is resumed after the passing of the joint of the webs, one of two procedures described below is usually taken in order to supply the liquid to the extruder head.
(a) The liquid feed pump is stopped. The liquid pump is started to supply liquid simultaneously with the start or with the resumption of the application of the liquid to the web.
(b) The pump is started before the liquid is applied to the web, so that the liquid flows out of the extruder head before it is applied to the web.
When following procedure (b), the top of the extruder head and vicinity thereof become contaminated because the liquid flows out of the extruder head. Since the magnetic liquid is made of an organic solvent, the magnetic liquid on the top of the extruder head dries quickly so that the dried matter clings as an extraneous substance thereto. This results in making a streak or the like on the surface of the film of the applied liquid. For that reason, it is the procedure (a) that is desirable to start or resume the application. However, the pump cannot be driven at a desired rotational frequency the instant it is started. Therefore, it takes some time for the supplied liquid to reach a prescribed flow rate. For that reason, the liquid applied to the web cannot reach a required quantity as soon as the application of liquid is started or resumed. Before the applied liquid reaches the required quantity at the time of the starting or resumption, the corresponding portion of the product made by the application becomes defective. This leads to a poor yield as to the product. If the number of times that web joints are passed across the extruder head is large, the number of defective portions of the product is also large.